UFH Overlay System and Heat Pump Which Pipe Size?

GreenLeaves22

Hello, I am looking to install underfloor heating to the ground floor of my house. This will initially run off a boiler but I may switch to a hybrid system in the future with an air source heat pump. The UFH will go on top of a timber suspended floor which has insulation between the joists. This will be then tiled on top. I have looked at few overlay systems but not sure whether to go for a 12mm or 16mm UFH pipe. Does an air source heat pump need a 16mm pipe to run efficiently or will 12mm work as well? Thanks 

matelodave

I have an underlay system, installed 14 years ago, with 12mm pipe working with a heat pump and not had any problems. Although I guess that 16mm might be better and possibly provide more heat for the same area with similar flow temperatures.

But you'd need to do the sums to see what the difference may be and make sure that you will get adequate heat output at the flow temperatures that you are contemplating. We get away with 30-36 degrees unless its really cold when it can get up to 40 but we were fine even last week when the temps went down to -9.

We dont have a buffer tank or volumiser and the only minor thing I would suggest is be careful about to many zones. IMO our is over controlled with eight zones and as they close down it make the heatpump cycle more than it should. I have now disabled the zone controls and let the heatpump do its own thing using weather compensation.

We are more comfortable, the HP doesn't cycle anywhere near as much and our electricity bills are lower as its not continually restarting.

Update 

I've just had a shufti at the specifications of the PolyPipe Overlay system (which is what I've got) and it suggests that the 12mm stuff at 200mm pipe centres has an output of around 100W/m2 with mean water temp of 50C.

The also have an Overlay-plus version which has an output of 82W/m2 at 50Cmwt which appears to be 12mm pipe

Whereas 15mm pipe at 200mm centres has an output of 60W/m2 with a mean flow temp of 60C which to my mind (without doing all sorts of heat calculations) sounds a bit counter-intuitive.

As far as I can remember I think my pipe spacing is 150mm and it was originally installed with a fixed water temp of 40C.  Although after a lot of tweaking and setting weather compensation I got it down to between 30-35ish. so I guess our heat output is nearer 30-40w/m2 or even less.

Perhaps you need some advice from the suppliers to clarify the heat outputs and how to derate it  at lower mean water temps if thats your aim.

GreenLeaves22

Thanks very much for taking the time to reply. I’ll  speak to the suppliers. 

Reed_Richards

I'm far from an expert on UFH but one thing I have picked up on is that pipe density is important.  @matelodave alludes to this with his reference to pipe centres.  The smaller you can make those, the more heat output you get per unit area so the cooler you can run your UFH.

matelodave

theres some useful info here  - https://ambienteufh.co.uk/app/uploads/2021/10/Ambiente-HeatOutput-Table-Doc-21.pdf